© Boardworks Ltd 20091 of 45. 2 of 45© Boardworks Ltd 2009.

2 of 45 © Boardworks Ltd 2009


Light moves in straight lines


reflected rayincident ray

i r

angle of incidence (i) = angle of reflection (r)

This is called the law of reflection and is true for any type of wave being reflected from a surface.

normal

plane mirror

The law of reflection

point of incidence

When a light ray hits a mirror it changes direction: the ray is reflected.


The law of reflection in action


Many images are enlarged or reduced versions of the object. The extent to which an image’s size differs from an object’s is known as the magnification.

Describing images – size

Many devices take advantage of the ability of mirrors and lenses to alter the size of an image.

These include projectors, microscopes and binoculars.


Describing images – orientation

If the rays of light from the top and bottom of an object cross over before an image is formed, the image will appear upside-down.

Inversion can also occur if rays from the right and left of an object cross over. This is known as lateral inversion and is seen most commonly in plane mirrors.

This is an inverted image.


When we look into a mirror we see an image. The image appears to be behind the mirror.

Describing images – real or virtual?

If you look behind the mirror, the image is obviously not there, so we say it is a virtual image. A virtual image is one which cannot be formed on a screen.

A real image is one that can be formed on a screen, such as the real image from the projector, which you are reading now!


Describing images – quiz


cat

If we look into a mirror, we see an image.

What kind of image is formed in the plane mirror?

laterally inverted same size as the object virtual

Images in plane mirrors


Convex mirrors are curved so that they bulge outwards.

F

ƒ = focal length

Convex mirrors

Convex mirrors are diverging mirrors. They reflect rays of light away from a focal point (F) which lies behind the mirror.

Rays parallel to the mirror’s central axis are reflected so that they appear to have come from this focal point.


Images in convex mirrors


Concave mirrors are converging mirrors, as they reflect rays of light towards a focal point (F).

F

ƒ

Concave mirrors

If a light source is placed at the focal point, the mirror will produce a beam of parallel light rays.

The distance between the mirror and the focal point is called the focal length (ƒ).ƒ becomes smaller as the mirror’s curve increases.


ƒ

Choose a distant object to get parallel rays of light.

Move the concavemirror back and forward to produce a clear image on a screen.

Use a ruler to measure the distance between the mirror and screen. This is the focal length (ƒ).

Finding f of a concave mirror


Images in concave mirrors


Concave mirror summary

When the object is beyond the focal point of the mirror the image is real and inverted. Its size varies depending on the object’s distance from F.

When the object is between the focal point and the mirror the image is large, virtual and not inverted.

When the object is on the focal point the is no image.

Can you explain why?


Spear fishing has been used for centuries and is still practiced by subsistence communities.

What is refraction?

As the light crosses the boundary between fluid and glass, it is bent, producinga distorted image.

The straw appears to be bent in the liquid. What is causing this effect?

This known as refraction.

To accurately spear the fish,fishermen learn to aim a short distance behind the fishes’ image, in order to compensate for the effect of refraction.


Refraction in a glass block


If an incident ray enters glass at an angle, then it is refracted, and bends towards the normal. The angle of incidence (i) is larger than the angle of refraction (r).

A material which light passes through, such as glass or air, is known as a medium.

Refraction – labelling diagrams

When the light leaves the glass, the opposite happens: it bends away from the normal.

normal

normal

incidentray

refractedray


Refraction summary


Wavelength and speed effects


A model for refraction


Light travels at very high speeds. It reaches 300,000,000 m/s in a vacuum, and is marginally slower in air.

As the speed of light varies depending on the medium, different materials refract light by different amounts.

In other materials the speed of light varies significantly:

material speed of light (m/s)waterperspexglassdiamond

225,000,000200,000,000200,000,000120,000,000

Speed of light

This means that it takes light a mere eight minutes to reach the Earth from the Sun!


Refractive index is calculated by comparing speed of light in a vacuum to that in a given medium:

The speed of light in a vacuum is 300,000,000 m/s, and the speed of light in water is 225,000,000 m/s. What is the refractive index of water?

refractive index = 300,000,000 225,000,000

refractive index = speed of light in vacuum speed of light in medium

Refractive index is a measure of how much a substance slows down light. The higher its value, the more a medium slows light. The more the light is slowed, the more it bends towards the normal.

Refractive index

= 1.33


Use the information in the diagram to find the refractive index of glass.

refractive index (n) = sin i sin r

refractive index = sin 45° sin 28°

The refractive index can also be calculated using Snell’s Law, which uses the angle of incidence (i) and angle ofrefraction (r) to establish how much a medium slows light.

Snell’s Law

refractive index = 1.5

normal


Use Snell’s Law to answer the following:

sin r = sin i refractive index

Using Snell’s Law

Diamond has a refractive index of 2.4. If light passes into a diamond crystal at an angle of 15°, find the angle of refraction.

sin r = sin 15° = 0.12.4

r = sin-1 0.1

r = 6.2°


Using Snell’s Law

Perspex has a refractive index of 1.5. If a ray of light passing into a perspex block has an angle of refraction of 24°, find the angle of incidence.

sin i = sin r × refractive index


sin i = (sin 24°) × 1.5 = 0.61

i = sin-1 0.61

i = 37.6°


Using Snell’s Law

If a ray of light enters water at an angle of 15° and has an angle of refraction of 11.2°, find its refractive index.

refractive index = sin 15 sin 11.2


refractive index = 1.33


Total internal reflection

Total internal reflection is when a light ray hits the boundary between two materials of different densities, and is reflected rather than refracted.

1. The angle of incidence must be greater than the critical angle.

2. The light must be passing from a high refractive index to a low one.

Sometimes only part of a light ray will be reflected, while the rest crosses the boundary and is refracted.

There are two conditions for total internal reflection:


Total internal reflection – a recap


Optical fibres

Optical fibres are thin strands of solid glasswhich are widely used in communication, medicine, lighting and as sensors.

The glass core is often encased in a layer of cladding, which prevents light escaping the core. A protective plastic jacket surrounds the whole fibre.

Why are the materials used to make the core and cladding of an optical fibre important?

They exploit total internal reflection in order to carry beams of light over long distances and along winding paths.


How does the refractive index (n) of different materials affect the critical angle (c) at a boundary with air?

Critical angle in different materials

The greater the refractive index, the smaller the critical angle.

42°

49°

24°

1.5

1.33

2.4

medium n c

water

glass

diamond

50°1.31ice

40°1.54quartz

Different materials have different critical angles.


sin c = nr

ni

Each medium has a different critical angle. We can calculate the critical angle if we know the refractive index:

Calculating the critical angle

The critical angle varies depending on the refractive index (n) of both materials at a boundary.

What do you notice about this equation?

Calculate the critical angle of perspex at a perspex to air boundary.

sin c = 1 = 0.671.5

perspex: n = 1.5 air: n = 1

c = sin-1 0.67

c = 42°


Calculate the critical angle for this glass to water boundary.

sin c = 1.33 = 0.89 1.5

Calculating the critical angle – examples

Now repeat your calculation for an air to glass boundary.

When light hits a medium with a higher refractive index:

watern = 1.33

glassn = 1.5c = sin-1 0.89 c = 63°

As sin (x) has a maximum value of one, total internal reflection is impossible.

ni

nr

> 1


Total internal reflection – true or false?


A ray of white light can be split into a spectrum of colours. This is known as dispersion.

violetWhich colour is refracted the most?

Richard – red of – orange

gave – green battle – blue in – indigo vain – violet

The different colours of light have different wavelengths. The different wavelengths are refracted different amounts.

Prisms


Speed of light in materials


Dispersion – summary


Glossary


Anagrams


Reflection and refraction quiz

© Boardworks Ltd 20091 of 45. 2 of 45© Boardworks Ltd 2009.

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Transcript of © Boardworks Ltd 20091 of 45. 2 of 45© Boardworks Ltd 2009.